Online and Gaming: This is the most common context. Players will often beg an expert on their team, “大佬,带带我!” (dàlǎo, dài dài wǒ!), which means, “Master, please carry me!”
Workplace and Industry: It's used informally to show respect for a senior or highly skilled colleague. For example, “If you have a problem with this code, go ask Zhang Wei. He's the real 大佬 in our department.” It's a high compliment.
Hobbies and Arts: Anyone who demonstrates incredible skill in a hobby—like painting, photography, or music—can be called a `大佬` by admirers. “看这个视频,这位美妆大佬的技术太牛了!” (kàn zhège shìpín, zhè wèi měizhuāng dàlǎo de jìshù tài niú le!) - “Watch this video, this makeup guru's skills are incredible!”
Connotation: The connotation is overwhelmingly positive, respectful, and admiring. It is, however, strictly informal. You would not use it in a formal business presentation or academic paper.